A lysosome (derived from the Greek words red blood cells). They are structurally and chemically spherical vesicles containing hydroliticenzymes, which are capable of breaking down virtually all kinds of biomolecules, including proteins, nucleic acids, carbohydrates, lipids, and cellular debris. They are known to contain more than fifty different enzymes which are all active at an acidic environment of about pH 5. Thus they act as waste disposal system of the cell by digesting unwanted materials in the cytoplasm, both from outside of the cell and obsolete components inside the cell. For this function they are popularly referred to as "suicide bags" or "suicide sacs" of the cell. Further, lysosomes are responsible for cellular homeostasis for their involvements in secretion, plasma membrane repair, cell signalling and energy metabolism, which are related to health and diseases.[1] Depending on their functional activity their sizes can be very different, as the biggest ones can be more than 10 times bigger than the smallest ones.[2] They were discovered and named by Belgian biologist Christian de Duve, who eventually received the Nobel Prize in Physiology or Medicine in 1974.

The size of lysosomes varies from 0.1–1.2 μm.[10] At pH 4.8, the interior of the lysosomes is acidic compared to the slightly basic cytosol (pH 7.2). The lysosome maintains this pH differential by pumping in protons (H+ ions) from the cytosol across the membrane via proton pumps and chloride ion channels. Vacuolar H+-ATPases are responsible for transport of protons, while the counter transport of chloride ions is performed by ClC-7 Cl-/H+ antiporter. In this way a steady acidic environment is maintained.[11][12] The lysosomal membrane protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. The cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function well or at all in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not destroyed in case there is leakage of the hydrolytic enzymes from the lysosome.

Formation

Many components of animal cells are recycled by transferring them inside or embedded in sections of membrane. For instance, in endosomes.[13]

The production of lysosomal proteins suggests one method of lysosome sustainment. Lysosomal protein genes are transcribed in the nucleus. mRNA transcripts exit the nucleus into the cytosol, where they are translated by ribosomes. The nascent peptide chains are translocated into the rough endoplasmic reticulum, where they are modified. Upon exiting the endoplasmic reticulum and entering the Golgi apparatus via vesicular transport, a specific lysosomal tag, mannose 6-phosphate, is added to the peptides. The presence of these tags allow for binding to mannose 6-phosphate receptors in the Golgi apparatus, a phenomenon that is crucial for proper packaging into vesicles destined for the lysosomal system.[14]

Upon leaving the Golgi apparatus, the lysosomal enzyme-filled vesicle fuses with a late endosome, a relatively acidic organelle with an approximate pH of 5.5. This acidic environment causes dissociation of the lysosomal enzymes from the mannose 6-phosphate receptors. The enzymes are packed into vesicles for further transport to established lysosomes.[14] The late endosome itself can eventually grow into a mature lysosome, as evidenced by the transport of endosomal membrane components from the lysosomes back to the endosomes.[13]

Lysosomotropism

Weak bases with lipophilic properties accumulate in acidic intracellular compartments like lysosomes. While the plasma and lysosomal membranes are permeable for neutral and uncharged species of weak bases, the charged protonated species of weak bases do not permeate biomembranes and accumulate within lysosomes. The concentration within lysosomes may reach levels 100 to 1000 fold higher than extracellular concentrations. This phenomenon is called "lysosomotropism"[20] or "acid trapping". The amount of accumulation of lysosomotropic compounds may be estimated using a cell based mathematical model.[21]

A significant part of the clinically approved drugs are lipophilic weak bases with lysosomotropic properties. This explains a number of pharmacological properties of these drugs, such as high tissue-to-blood concentration gradients or long tissue elimination half-lifes; these properties have been found for drugs such as haloperidol,[22]levomepromazine,[23] and amantadine.[24] However, high tissue concentrations and long elimination half-lives are explained also by lipophilicity and absorption of drugs to fatty tissue structures. Important lysosomal enzymes, such as acid sphingomyelinase, may be inhibited by lysososomally accumulated drugs.[25][26] Such compounds are termed FIASMAs (functional inhibitor of acid sphingomyelinase)[27] and include for example fluoxetine, sertraline, or amitriptyline.

Controversy in botany

By scientific convention, the term lysosome is applied to those vesicular organelles only in animals, and vacuoles to plants, fungi and algae. Discoveries in plant cells since the 1970s started to challenge this definition. Plant vacuoles are found to be much more diverse in structure and function than previously thought.[28][29] Some vacuoles contain their own hydrolytic enzymes and perform the classic lysosomal activity, which is autophagy.[30][31][32] These vacuoles are therefore seen as fulfilling the role of the animal lysosome. Based on de Duve's description that “only when considered as part of a system involved directly or indirectly in intracellular digestion does the term lysosome describe a physiological unit”, some botanist strongly argued that these vacuoles are lysosomes.[33] However, this is not universally accepted as the vacuoles are strictly not similar to lysosomes, such as in their specific enzymes and lack of phagocytic functions.[34] Vacuoles do not have catabolic activity and do not undergo exocytosis as lysosomes do.[35]

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